The invention relates to a method of manufacturing a dispenser cathode in which an impregnated and sintered molding is embedded in a holder.
The invention also relates to a dispenser cathode manufactured according to said method.
Such dispenser cathodes are used in electron guns for television display tubes, picture pick-up tubes, travelling wave tubes, klystrons, and the like.
A dispenser cathode, in which a sintered porous tungsten molding which is impregnated with barium aluminate, is compressed in a holder which is clamped to the cathode shank, has been described in U.S. Pat. No. 3,467,879. The molding is laid in the holder in the form of a pill and is compressed by means of a cylindrical die. The holder has been obtained by deep drawing from a molybdenum sheet having a thickness of 100 .mu.m microns. The impregnated and sintered molding is clamped in the holder by deformation of the edge of said holder so that a relatively large gap is present between the molding and the holder.
The use of semiconductors in television cameras and television sets has resulted in the need for rapidly warming-up cathodes. These cathodes are cathodes which emit a sufficient amount of electrons, after less than 5 seconds from switching on, so as to be able to produce a picture on the display screen of the display tube. It is known inter alia from the Philips Product Note 67, Quick Vision CTV picture tube A66-410 X, that the warm-up time (t.sub.h) of a cathode is proportional to the ratio of the thermal capacity (C.sub.th) to the effective heating power (P.sub.e) of the heating body of the cathode. EQU t.sub.h .alpha.(C.sub.th /P.sub.e).
In order to be able to shorten the warm-up time of the known cathode described above, an improved cathode should be composed of smaller components than the conventional cathodes so as to reduce its thermal capacity. The drawback of this is, however, that the life of such a cathode is considerably reduced. As a matter of fact, the life is determined inter alia by the ratio between the quantity of impregnate present in the molding which is proportional to the volume of the molding and the quantity of reaction products of the impregnate which leave the molding by evaporation from the surface of the molding. The latter quantity is proportional to the surface area of the molding. It is thus obvious that reducing the size of the molding results in shortening its life.
It has been found that the reduction in life is in fact even larger than follows from the above considerations. This is because the exhaustion of the cathode is diffusionlimited. It has therefore been found that a cathode, of the known, above described construction and of the rapidly warming-up type (less than 5 seconds) which is operated at a voltage 11% above the usual voltage, has a life of only 5000 hours. An advantage of a cathode of the known construction is that the impregnated and sintered molding, prior to being placed in the holder, can be tested for its composition, for example by accurate weighing.
U.S. Pat. No. 3,623,198 describes a method which does not have this advantage. In this application, the material, from which the molding is manufactured, is provided in powdered form and then is pressed by means of a compression tool, into the hollow end face of a cathode shank to form the molding. Subsequently the molding is sintered. This method has been found to be unsatisfactory in manufacturing small cathodes of the rapidly warming-up type, since the deformation of the end face necessary to make it hollow is, for a very small cathode shank, substantially impossible.